Maryam Mombeini attempted to fly out of Tehran on Wednesday evening with her sons Ramin and Mehran but airport security prevented her joining them on board the Lufthansa flight for Canada via Germany.
"Enough is enough," Ramin Seyed Emami, a well-known singer, said on his Instagram account alongside a picture of him and his brother on the plane without their mother.
He warned the Iranian authorities not to "think that my brother and I will stay silent for one second until we are reunited with our mom."
Environmentalist Kavous Seyed Emami, 63, died in prison in February after being arrested on espionage charges the previous month along with seven other members of the Persian Heritage Wildlife Foundation he founded.
The authorities say he committed suicide, but the family have questioned the verdict and say they were threatened by security forces.
Canada, which has repeatedly sought answers from Iran about the circumstances surrounding Emami's detention and death, demanded that his widow be allowed to leave.
"We demand that, as a Canadian, she be given the freedom to return home."
Canada severed diplomatic relations in 2012 at the height of the standoff between the West and Iran over its nuclear programme.
Iran does not recognise multiple nationality and treats all joint citizens as Iranians.
Iranian prosecutors allege that Emami's foundation installed cameras in strategic areas under cover of filming their wildlife, while in fact monitoring the country's missile activities.
One of the NGO's key projects was monitoring the endangered Asiatic cheetah, which meant they operated across large swathes of the northern province of Semnan, home to military sites and missile-testing grounds.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)